This invention relates to a reflection preventing film which is useful for microprocessing in lithographic processes using various radiations, and to a process for forming a resist pattern using the reflection preventing film.
In the field of the production of integrated circuits, a lithographic process which enables fine processing in an order of subhalf micron is now being developed in order to obtain a higher degree of integration.
In this lithographic process, a desired pattern is obtained by coating a resist such as a positive type resist consisting of a novolak resin and a quinonediazide sensitizer usually in a thickness of 0.5-10 .mu.m on a substrate, transferring the master pattern thereto using a stepper and then developing the same with a suitable developing solution.
However, radiations to be used in the process have a single wavelength, and hence, the incident radiation and the radiation reflected at the upper and lower interfaces of a resist film interfere with each other. As a result, there occurs a phenomenon called "standing wave effect" or "multiple interfering effect", namely the phenomenon that even if the amount of radiation applied is constant, the effective amount of radiation applied to the resist film is varied by the interference of the radiations with each other in the resist film when the thickness of the resist film is fluctuated, whereby the formation of a resist pattern is adversely affected in some cases. For example, when a slight change in composition and viscosity of resist and conditions for coating a resist results in a change in coating film thickness, or the presence of a step in the substrate results in a difference in coating film thickness (the thickness in concave portion becomes larger than that in convex portion), the effective amount of radiation applied is varied depending upon the film thickness thereof, the pattern dimension is varied and the pattern dimension precision is lowered.
In order to solve such problems, a process has heretofore been proposed which comprises forming a reflection preventing film on a resist film to inhibit the reflection at the resist film interface, thereby decreasing the multiple interference in the film [see Japanese Patent Application Kokai No. 60-38,821, J. Electrochem. Soc., Vol. 137, No. 12, 3900 (1990) and the like]. In this case, the reflection-inhibiting effect at the resist interface depends mainly upon the refractive index of reflection preventing film and the film thickness. The refractive index of an ideal reflection preventing film is .sqroot.n in which n is the refractive index of the resist. For example, where the resist is a novolak resin type resist (n=1.64), the ideal refractive index of a reflection preventing film is 1.28. The film thickness of an ideal reflection preventing film is odd time the .lambda./4m in which .lambda. is the wavelength of a radiation and m is the refractive index of the reflection preventing film.
As the material for reflection preventing film, there have been known polysiloxane, polyethyl vinyl ether and polyvinyl alcohol. However, a reflection preventing film consisting of one of these materials has the following problems: (i) Since the difference in refractive index from resist is small, the standing wave effect cannot be sufficiently inhibited (the basic problem). (ii) When combined with a certain resist, the material is slightly mixed with the resist (this is called intermixing) and modifies the resist, whereby such resist performances as resolution, pattern shape and the like are deteriorated. (iii) When a reflection preventing film insoluble in water or a developing solution is used, it is necessary to remove the film with a reflection preventing film removing agent separately before the development. (iv) Even when polyethyl vinyl ether or polyvinyl alcohol is used as a reflection preventing film, it cannot always be said that the film is sufficient in solubility in water or a developing solution, and a residue remains on the resist, and in some cases, resist performances such as resolution, developability, pattern form and the like are deteriorated.